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What makes a bomber able to deliver nuclear bombs?
Currently the German Luftwaffe has Panavia Tornados to deliver American nuclear bombs in case of war. The bombers are getting older and are currently modernized. The German Luftwaffe also has Eurofighter Typhoons which are said to not be able to deliver nuclear weapons. Why is that so? (Same goes for the F-35 and the F-22)
AFAIK all that's needed for an aircraft to deliver nuclear bombs is

The ability to actually carry the bombs, which probably conform to some standard "form factor".
Some special electronics for arming the bombs, which must be installed into the bomber - this probably also follows some standards.

Is that all? I seem to remember some special aerobatic maneuver the aircraft should be able to perform to allow the survival of the crew. Does that exist and is it a problem for modern fighter/bomber aircraft?

They also need to get away from the nuclear blast. Otherwise it's a suicide mission. That maneuver is a loop, not all aircraft can do it.

@ratchetfreak  Not true.  It could be a stand-off weapon.  Even some helicopters can drop nukes.

Also, by the point you're dropping nukes you have a fairly low life expectancy anyways.

For the F-22, it's that the standard US nuclear bomb (the B61) literally does not fit in the F-22 weapons bay.

@cpast -- is it not compatible with external stores either? :P (not that you'd really want to do that, but just saying)

@UnrecognizedFallingObject I'm not sure the external stores have the failsafes required for nuclear weapons.

@cpast -- good point, those safeties might just not be routed to a pylon (I think that you can put a nuke on a pylon on at least some aircraft; the B-52 comes to mind)

@UnrecognizedFallingObject You can, but you need to also ensure that the pylon won't come off without being commanded to, including a lock which is independent from the normal pylon-jettison circuits.

@cpast: I've read that, too. But these bombs could be redesigned.

External stores on an F22 circumvent its stealth characteristics, increasing the "suicide mission" status.

@Smithers: Given that most airplanes capable of carrying nukes are not "stealth", that statement seems a little too "easy" to me.

@MartinSchröder It really, really depends. If it doesn't require modifying the physics package (the "nuclear bomb" part of a nuclear bomb), then maybe (it's still expensive to modify nuclear bombs, but it's doable and they're doing it for the F-35). If the size constraints rule out the current physics package, there's a chance it'd be cheaper to redesign and modify F-22s than to design a new nuclear bomb from scratch (nuclear bomb design is extremely complicated), and it's certain to be less controversial; the F-22 is likely what'd be modified there, if it had needed to carry them.

You could probably ditch the special electronics as well, if necessary, with some jumpers in the right places.

You are right in saying that's all that's needed, with the addition of a strategic or tactical need.
The need for a "special evasion" technique is only needed for aircraft dropping megaton range weapons in an air burst.  Kiloton range weapons could be dropped from altitude with no special evasion required except a lot of speed.  Sub-sonic aircraft would need to be "frisky" in their escape. Read about Tsar Bomba for a good example which demonstrates that even with the biggest bomb ever dropped, a relatively slow, lumbering aircraft can escape.
Even helicopters can drop nukes.
Most aircraft are only tasked to drop tactical, low yield weapons.  The real madness is assigned to subs and missiles.
As the comment above says, I suspect that most crews dropping strategic weapons will not care much whether they survive or not.

The technical term for the maneuver in question is toss bombing

@Mark One possibility, but only if delivered at low level. Mid-high altitude is just a fast turn and run away.

That's not quite all; you also need coordination with the people who own the nukes, and that seems to be a big sticking point here (the Germans don't want to share sensitive aircraft technical info with the US, which would be needed for the US to certify the Typhoon to carry our nukes).

@cpast  Good point.  I am assuming use by the "owning" nation.

Why performing a loop and not going straight ahead. Looping would makes you loose some precious speed needed to escape...

@Antzi One reason I can think of immediately is that a bomb starts out with the same velocity as the bomber. The bomb isn't just going straight down, but also forwards. So, it's probably better to be moving the opposite direction as what the bomb is moving. I'm not sure how quickly drag will reduce the forward speed of the bomb, though.

@reirab Ignoring the difference in drag between some forward flight and a vertical fall, the time taken to drop is exactly the same no matter what the launch velocity.

@Simon Yes, but the lateral position of the detonation isn't. If you're trying to get away from the blast, it's usually better to move the opposite direction as the bomb rather than the same direction.

@reirab Absolutely.  Sharp turn, and run - but not a "loop" or toss bomb.

The toss bomb actually throws the bomb forward and UP which is something others have missed pointing out here, that coupled with the fact that most bombs dropped this way are also slightly retarded in free fall by a small parachute means that you have more time to get away then straight and level flight, and you can 'toss' the bomb further from your airplane. This tactic is ALSO used to avoid heavily defended locations, as the bomb continues on a upward/forward track and can travel a greater distance, potentially letting you toss from outside AA ranges.

In the light attack community we had the loft and the laydown available to us. The loft (toss was not used in our community, although it might have been used in others) was done at 3 miles and we never saw the target, bomb coming off the aircraft at around 42 degrees nose high. It was an Immelaman. The laydown was go as fast as you can, as low as you can, and run away as fast as you can. I never got the idea that I would be coming back home.

Simon's answer discusses technical requirements to deploy nuclear weapons on an aircraft. However, there are less-technical requirements as well. Specifically, for reasons that should be fairly obvious, countries are very concerned that the aircraft carrying their nuclear weapons will not release an armed weapon when they aren't supposed to. At least for the US, just about anything involved with nuclear weapons has to be certified to ensure that it won't cause a nuclear accident. This involves lots of failsafes; for instance, an aircraft must have a locking system which is entirely separate from the release mechanism and which physically restrains the release mechanism, so that no failure in the release mechanism alone can result in release.
Actually complying with the requirements largely falls within a broadened version of "electronics to deal with the bomb" (it also has some additional mechanical stuff required, and certain preexisting parts may need additional failsafes added, but it's in the spirit of that question, and in any case it's not that expensive). However, certification and preparation of the Typhoon would involve US personnel learning lots of technical details of the plane, which Eurofighter would rather the US didn't (after all, they want to export Eurofighter). This isn't an issue for most munitions, because normally a country owns the weapons it deploys; nuclear weapons in Germany are different, because Germany is a non-nuclear weapons state that is forbidden from having nukes in peacetime under the Nuclear Nonproliferation Treaty. Nukes in Germany are under US control, and Germany only gets to deploy them if the US lets them during a war, so US regulations apply, and the added requirement is "a nuclear weapons state lets you use their bombs." In addition, there's a requirement of "the government is willing to carry nukes;" if the German government decides they don't like the idea of carrying nukes in wartime, the plane won't get the needed modifications.

An example of an accident suspected of coming close to a detonation: 1961 Goldsboro B-52 crash

The willingness of the german government to carry the nukes is difficult to gauge: All german foreign ministers since 1999 have publicly expressed their desire to remove these weapons from Germany. At the same time these governments have prepared lifetime extensions for the Tornados (current EOL is at least 2030). I believe most german politicians hope that this issue will go away when the current generation of nukes is EOLed - but now the USA are planning new nukes...

What neither of the other answers have touched on is the real reason why some aircraft can be used to deploy nuclear weapons and other can't.
And that is the special equipment needed on board these aircraft to carry, arm and and launch the weapons. Which is a big reason not just why some aircraft can and others can't deploy nuclear weapons but pretty much any weapon.

It's not as if you can just bolt a bomb or missile to an aircraft, you need the correct connectors/clamps to attach it to, you need the right electrical, mechanical, and/or electronic equipment to arm it, give it guidance information if applicable, things like that.
And in case of nuclear weapons there are safeguards built in that require a very special set of equipment to operate and enter launch codes (in the first generations of nuclear weapons it even required a crew member to crawl into the bomb bay of the aircraft in flight to insert the trigger mechanism into bomb by hand).

While the connectors on the launch rails have in part been standardised across fleets, there are still differences, and in guidance systems there are of course far more differences still (to launch an AMRAAM you of course will need different data to the missile than to launch a Harpoon for example).

That's what I mean with "special electronics for arming".

@MartinSchröder yes, you need those, and that means space and power for them. And with space at a premium that's often not installed.

You forget to mention the software requirements, as well. Bomb Weights and shapes and flight times are all carefully combined to be dropped on target via the Aircrafts avionics, if they aren't setup correctly on the aircraft you won't hit what you aim at (though this is a minor issue with something as damaging as a Nuke).

@Mark true, though that's implicitly covered under "electronics". A lot of work has been done to make nuclear weapons rather accurate btw, in order to enable smaller warheads to take out heavily hardened targets.

I can provide information from the operational side of things, although I cannot say much about the engineering side of hanging a nuclear weapon on the belly of an aircraft.
The A7E was a US naval light attack bomber capable of carrying up to four nuclear bombs under its wing. The bombs it could carry were the B28, B57, and the B61. They were tactical nukes in the kiloton range. The A7E has been retired, but replaced the A4 in Vietnam, and then served as the light attack delivery platform for the US Navy until finally being replaced by the FA-18 in the mid to late 1980's.
The idea of using a kiloton range nuclear device over conventional bombs was described to me at one point. Let's say you are going to war, no, that you are seriously going to go to war, then when you hit an airfield you want to disable it. If you have limited assets it might be considered that using a tactical nuke is the best alternative. One bomb, one aircraft, and if they make it to the target, a hole in the runway that will not be fixed quickly, and of course all without the messiness of a megaton range bomb.
My experience with these payloads was as a loading officer and a delivery pilot. During my tenure at the squadron, which was 3 years, we only dropped one practice nuke, and this was done start-to-finish from the deck of the carrier. The A7E was loaded under the deck, brought up on the elevator, the pilot got in, took a catapult shot, delivered the bomb, and came back to take a three wire. By the way there were no "practice nukes," and every nuke we handled was the real one. It was a mindset.
Really, the only requirement that I saw from a loading officer or pilots perspective, was that you can hang the bomb under my wing. If you hung it, I could arm it. I do not remember any special cockpit procedures, except wear your eye patch. Other than that, you want the bomb on target, and so you want a platform that can deliver it, not too worried about it getting home though. Most bombing platforms today have that sort of technology. The point about arming is a good point, and I am not completely sure about my cockpit procedures. It was a long time ago, and anyway things have changed. My guess is that there were no special arming requirements, a pulse of electricity was delivered to the station under the wing, which activated the release. No other bombs had any special arming considerations. Essentially, a charge was initiated and a boot kicked the ordinance off the wing.
Why can some tactical jets carry such a weapon and others can't? I am not sure about this, but some of the considerations are payload weight and physical constraints of the delivery aircraft, e.g. wing size, bomb bay size, etc. There are also characteristics of the bomb as it departs the aircraft. If it has aerodynamic modes (because it is flying) where it comes back up to the aircraft and collides then it will not be allowed under the wing. So I would speculate it has more to do with airframe aerodynamics and physical constraints, and less to do with electronics and arming.
I also was on the standing mission planning team aboard the ship that considered the strategic and tactical issues the pilot would face in a given delivery. Don't want to talk about the sort of intelligence we were given to make our decisions. There wasn't any thing out of the ordinary for the mission planning folder that would be handed to the pilot when the time came. Just a bombing mission planned in advance and reviewed by the upper echelon.
I will make this comment. In the A7E we were given a packet which included a kneeboard chart, the navigation charts for the low level, and an eye patch. Don't remember much else in it that concerned me. The chart gave me the path in, the path out, and the location of the carrier to come back to. We always joked about coming back to the carrier, as if it was going to be where it said it was. Also we had ways of entering the airspace of the ship without radio communications so you wouldn't get shot down as a threat. That too worried us as pilots. But the eye patch? So I asked about that. Here is what I was told:

Your route to target will be deconflicted with other targets, i.e.
  other tactical nukes going off at different locations and times in
  your forward hemisphere before your own delivery. On the chance that
  one does goes off at the wrong time we have provided you with an eye
  patch. Put the eye patch on before feet dry, and if you find yourself
  blind, take it off and fly with the redundant eye to your target and
  deliver your weapon. See you when you get home.

Okay. I get it, we are really serious about this! This brings up the last point I wanted to make. We always delivered our nukes in a manner that was different than conventional bombs. It was about getting away from the blast zone as quickly as possible, and in particular the electro-magnetic pulse, which would render us virtually helpless. The electronic fuel transfer switches would be fried and the available fuel left in the various tanks wouldn't get you very far. Not to mention all the navigation systems and such that would be inoperable. At the time I was flying missions we used the loft and the laydown deliveries, which purportedly allowed us to survive the detonation. The loft was at 3 miles, pull up into an Immelman with 1 G. After completing the Immelman depart back to the ship. The laydown was delivered at 200 to 500 feet, above the power lines crisscrossing a country, as fast as you could go, with an exit straight away from the target. Still, that fast and low, hard to see the target. This helps to isolate you from threats inbound and outbound. Each delivery had its benefits and risks.

Primary requirements for delivery of Nuclear weapons beyonds the physical ability to transport the munition is the Arming and Fuzing capability. This extends to Authorization and control operations of the device, safety is integrated in each section and independently also.
For example control and authorization required interface via a PAL (Permissive Action Links) This is a physical device, combination lock or a electronic keypad-activated interlock.The intent is to prevent detonation without direct positive action instructing such.
Along with the ESD systems which are another set of safety systems the aircraft must be able to communicate and execute the require actions demanded of the bombs safety operations SOPS and systems.
That is the the only requirements to carry nuclear weapons and basically covered in the below areas.  Due to their awesome power and demands of failsafe carriage n flawless detonation (else the enemy can utilize the undetonated weapon) additional delivery platform hardware is required as for smart weapons of conventional types.
Also regarding the F-35, the air force intends these to be dual capable and will work to ensure its compatibility with the BP61 replacing F15 & F16 in this assignment.

US Army had a special type of nuclear explosive, they called: 'Demolition weapon',  or something like, its power was in the range of kilotons, had a weight of around 60 kg (133 pounds), and was discontinued by 1950. So, you can drop a nuclear weapon, if you have it, from any two seat ultralight airplane, and escape if you fly high enough. Plutonium can be obtained from a homemade reactor not bigger than a metal grain silo, they had one like this in the Barcelona University.
Fighting airplanes' technology is to ultralight aircraft what Formula 1 cars technology is to mopeds, it's hard to imagine that the aerial nuclear war procedures and capabilities are known to anybody out some restricted military audiences, and the features and performances of airplanes able to carry and launch nuclear weaponry can be watched at air shows and specialized publications, the USSR said that most of the technology information they got from the free world came from open access and purchase publications, probably, no government will sell a military technology to a third party if they don't have in reserve at home something better, it would be silly selling a technology that someone could use against you, as the experience is that big changes in government's lines are a true possibility

Not if surviving is part of your plan. Of course, nearly any size airplane could be designed to carry a nuclear bomb (though dropping it would be suicide for many of them, especially the ultralights you mention,) but I think the OP was asking primarily about why military aircraft are designated as being able to carry nuclear weapons or not. As the other answers have stated, this comes down to needing the proper safety mechanisms and the capacity to carry the form factor of the bomb(s) to which you have access.


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How are Virgin Galactic's SpaceShipTwo's windows so large?
From Virgin Galactic on SpaceShipTwo:

Windows in the passenger compartment will be 13 inches (33 centimeters) wide by 17 inches (43 centimeters) tall.

IMO that's enormous for an airplane, especially for a spaceplane! But, wouldn't such large windows cause problems? The Concorde had particularly small windows (passengers say that it's almost passport book sized) to maintain the integrity of the airframe and cabin pressure if a window happened to crack at 60,000 feet AMSL.
But wouldn't such a large window cracking at 110 km AMSL on SpaceShipTwo definitely cause cabin pressure to equalize to atmospheric pressure in seconds? What redundancy does SS2 have to prevent catastrophe in such an event?

the windshield of a spaceshuttle is also pretty large

But the astronauts have the option of wearing space suits! (but space suits are useless if not worn during event of sudden decompression). Virgin Galactic SS2 will require shirt-sleeves only.

Concorde is not really a good example - that technology was a long, long time ago.

IMO the Concorde's idea was very simple and failsafe. The area of the outflow valves matches the area of two windows, so if two windows blew out, you could just close outflow valves and retain (most) of the cabin pressure at altitude.

My guess?  They are really, really thick (and very possibly have two windows just in case one cracks).  And they want them big because this is a sight-seeing flight!

Here are a few ideas:

First and foremost, we're not handling metals here. Composites have the advantage of being able to be made stronger by adding on a few layers around high-stress area like windows. This was difficult with metals since this needed some sort of fasteners or adhesives to do. This reinforcement is seen in the photo below.
The windows are designed to keep an even pressure all around in the context of the fuselage shape at that point. You'll notice the flange design which keeps the fuselage shape and stop it from popping open. This is a little like the thicker part of a bottle neck. This is very difficult (if possible) to do well in a metal fuselage to this extent.

(source: msn.com)

The usage is completely different. Concorde was a commercial aircraft, intended for many years of operation. This is a very low-cycle aircraft which will see a fraction of that flying. The number of pressurisation cycles is a lot lower, putting much less force on the windows.
As for safety, the style of flying is completely different. Probably flown by test pilots, the aircraft might never even experience rain and will fly under very predictable conditions, with very few flight cycles.
Periods of elevated temperatures gradually degrades many materials, this is much less a concern with short flights. If I gather correctly this is the main issue with sustained high-speed flight, as seen on Concorde and SR-71 and other aircraft.

(source: guardian.co.uk)

Composites have a lot lower thermal expansion (CFRP vs. Aluminium is an order of magnitude lower) which reduces issues with expansion in the fuselage.
Concorde was a commercial aircraft with weight and operating price in mind. This aircraft is built for this purpose, with the windows considered as a main feature from the start. You could probably have been able to make the windows a lot larger on Concorde, at the unacceptable price of weight, production and operating cost.
Needless to say that knowledge of materials and structures has advanced substantially, all our these features can be modelled in detail on computers, not necessitating the traditional conservative approach.
As for redundancy, predecessor SpaceShipOne had double pane windows and double seals everywhere. For the passengers without suits, if a window were to blow entirely, you're most probably out of luck.

@JanHudec there might be a seal failure or similar. Not catastrophic. I don't see why they would take the chance to go without one. I agree it's pretty dangerous for the high altitude segments though. Passengers aren't going to wear pressure suits though.

"I'd guess the pilot has his mask on the whole flight.": Pressure masks are not sufficient above around 14km/46000ft (and that's most of the flight). Above that altitude, the total pressure is less than the normal partial pressure oxygen in human blood (14kPa), so breathing pure oxygen above 14km is not sufficient to keep one's blood saturated with oxygen. Above about 19km, the boiling point of water drops below 37°C causing further severe problems with various body liquids boiling (blood will not boil, because it has 8-10kPa higher pressure).

@JanHudec: Masks can work above 46000 feet AMSL. Again, Concorde had CPAP masks to deliver air to the pilots. It must've been uncomfortable to use!

@Manfred: this is a good answer and +1. I'm interested to know, why did you say "the aircraft might never even experience rain and will fly under very predictable conditions, with very few flight cycles." Is rain highly detrimental to performance and structural integrity of supersonic aircraft? Also, would the Concorde have worked as well as it did if it was made of CFRP (i.e., in terms of fuel economy, lifetime number of pressurisation cycles, and sustained Mach 2 performance)?

@shortstheory what dictates safety margins is how good control you can keep over the use. Concorde had to include safety margins for rain, ice, etc. that could certainly be expected. If used normally, it's a question of time before the huge windows would be scratched or damaged and would have to be changed.

@shortstheory
Water is of course fine in CFRP aircraft if it doesn't enter the composite where it may degrade down the resin, but this is no bigger problem than metal corrosion.
It built today, primarily better engines would save a lot of fuel. Concorde was 27 years old when retired. Supersonic composites is used on for instance the F35, so I can't see why a new Concorde would have problems. Metals were a pain as well due to thermal expansion. Lifetime I think should no issue as well. I'm no certainly no expert at composites, but I see no reason why it would not work.

I'm delighted to hear that! I hope we can get the next Concorde within our lifetimes! Again, thanks for the answer!

@JanHudec: The partial pressure of oxygen can safely go at least a bit lower than 14 kPA, since even non-acclimated humans can go as high as 8 kft (where the partial pressure of oxygen is considerably lower than at sea level) without any issues (which is why metal aircraft are pressurised to 8 kft cabin altitude, rather than MSL).

I think ultimately it's a business question.  Virgin Galactic is selling tours at a premium price where as Concorde was sold to move people from point a to point b.
With Concorde, since the point of the craft was to get people from point A to point B very very quickly, they put money into big engines and efficient aerodynamics.  The view doesn't really matter in that equation, but you do need structural integrity.  So they went with the cheapest way to get that structural integrity, small windows.
Virgin Galactic, on the other hand, is selling an experience.  And frankly, if you're going into space the two biggest selling points are going to be zero gravity and, honestly, the view.  Hence, even though it will make the craft more expensive to reinforce properly, they are installing very large windows.
As a note, airliner manufacturers could also install big old huge windows if they wanted as well, but do you want to pay another $50 a flight just to have a bigger window?  For most people the answer is no...hence small windows.
Basically it's just a matter of figuring out what you are trying to sell and, then, putting money into that selling point.  It's what gets you a good return on your investment :).
PS- lol, and after all that writing I appear to have answered the "why" and not the "how".  Well, let the internet judge me as it will, I'll leave this answer up for now :).

For what it's worth, the windows on the Boeing 787 are even bigger than the ones referenced here (19" tall): http://gizmodo.com/5829395/how-boeings-magical-787-dreamliner-windows-work

Yeah, the tech changes aren't really what I was trying to speak too, hence my final little note there...  But yeah, as the tech has improved you can make bigger windows for the same price as the smaller ones of yesteryear.

The Concorde had a lot of thermal expansion due to air/skin friction, that may have impacted window size.  SpaceShipTwo travels at much lower speeds, and doesn't have that issue.
Ultimately, the issue is whether the window is designed to withstand the pressure differential of cabin to space.  As long as it is thick enough, made out of strong enough materials, they should work safely.

Good point, Space ship 2 is a composite airplane so won't have the issues of thermal expansion like a metal airplane does.

According to this diagram, it looks like the Concorde actually travelled slower than the SpaceShipTwo.

What!? SS2 travels at Mach 4 through the atmosphere, and the Concorde only reaches Mach 2.04. SS2 is much faster than the Concorde.

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Does the wake from horizontal stabilizers merge with the wing wake?
There is already a long series of questions about the downwash and tip vortexes with nice images, but I'm going to add another one...
Large airliners (A380, B747-8) have horizontal stabilizers competing with small Boeing wings, and definitely larger than business jet wings.

Source
How does the tail wake look like when the horizontal stabilizers produce significant lift? Does it merge with the wake from the wing or is there a second wake above the first one?
Edit: I've found this image:

Source
Which shows the tail vortexes below the wing vortexes, and apparently larger than them.

Oops - what looks like a tail vortex to you looks like the contrails of the inner engines to me.

Yes, when viewed from far enough away.
Close up, both will have their own developing wake. The boundary layer leaving wing and tail will leave a speed discontinuity, and the downward moving wing wake will be complemented by a tail wake, which will slow down this downward movement directly behind the tail, and increase it left and right of the central tail wake. Normally, the horizontal tails of transport aircraft produce a considerable downforce during landing to compensate for the backward shift of the center of pressure when the high lift devices are employed.

Aircraft model flying through illuminated smokescreen (picture source)
This animated GIF shows the wake of the tail as a darker box inside the bigger wing wake. It forms two small vortices which rotate against the rotation direction of the wing wake and get spread out when being absorbed by the wing wake.
Friction will soon dissipate the much weaker tail wake, so at some distance all what is left is a single wake which looks slightly distorted around its center, when compared to the wake of the isolated wing.
Fun fact: The negative lift on the tail can even contribute thrust.

Awesome GIF, I'm mesmerized; have watched it like 100 times. Can almost see the opposite rotating vortices off of the tail, but its almost instantly destroyed by the churning of the wing-tip vortices and downwash.

@Devil07: Hehe, you made the same experience as I did. The tail vortex is hard to see, but is real.

If a picture is worth 1000 words, this GIF is worth 10,000 words. ;)

@CamilleGoudeseune: Thank you for the edit!

Heh.  Thank you for the pretty answer!

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Have pilots encountered a UFO, and what to do if you have?
Have pilots ever encountered a UFO? What guidelines, if any, are in place for times when this happens?

A related question about UFO's can be found over on skeptics.SE

Asking for anecdotes is a bad idea.

Never seen one, but if I do my plan is to say: "Gort, Klaatu berada nikto."

@TylerDurden That only works if you're holding the Book of the Dead.

Pilots do encounter UFOs all the time. I just want to emphasize that UFO means "Unidentified Flying Object" - something which is flying, and which the pilot cannot identify. It does not mean an alien craft.
The guidelines for pilots are as follows (AIM 7-6-4):

7-6-4. Unidentified Flying Object (UFO) Reports
a. Persons wanting to report UFO/unexplained phenomena activity should contact a UFO/unexplained phenomena reporting data collection center, such as Bigelow Aerospace Advanced Space Studies (BAASS) (voice: 1-877-979-7444 or e-mail: Reporting@baass.org), the National UFO Reporting Center, etc.
b. If concern is expressed that life or property might be endangered, report the activity to the local law enforcement department.

Air traffic controllers have the following guidance under FAA Order 7110.65 section 9-8-1:

9-8-1. GENERAL
a. Persons wanting to report UFO/unexplained phenomena activity should contact a UFO/ unexplained phenomena reporting data collection center, such  the National UFO Reporting Center, etc.
b. If concern is expressed that life or property might be endangered, report the activity to the local law enforcement department.

The question "Was it of the E.T. kind" is beyond the scope of this site. This subject is, let's just say, suitably covered by other sites on the Internet at large.
One notable example of a UFO report is the 2006 O'Hare UFO sighting. For other examples, a Google search for "pilot UFO anecdotes" offers some decent results. "ATC UFO report" even links to some recordings of pilot UFO reports to ATC.
Some objects commonly reported (genuinely) as UFOs are spacecraft, fast or high-flying military aircraft like the U-2 or the SR-71 when it was in service, rocket launches, weather balloons, kites, errant helium balloons from kids' birthday parties, flocks of birds, con trails, and lightning storms. Any of these and more can easily look, under the right circumstances, to a pilot like a "UFO."
The closest experience I've had in my flying history to a "UFO" happened at night, during a landing approach in the midwest U.S. I was on a downwind leg when a very bright light appeared above me in the center of the windscreen, and rapidly moved downward toward the horizon. It scared me half to death; my instinctual thought was that it was the landing light of another plane descending on top of me. It turned out to be a meteorite. It only took me a second or two to figure it out, but boy did it get my adrenaline up! There was no need to report it though.
Culturally, pilots are generally apprehensive about reporting UFOs, out of fear of sounding like a lunatic or unhinged conspiracy theorist and being laughed at. Nonetheless, reports are not uncommon.

Thanks for your answer and edit @dvnrrs I will come back to +1 when it will let me. I am glad you decided to add that extra bit to your answer; I know I was stretching it a bit re the "et kind", :-)


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How many pilots are licensed in Hong Kong?
Title says it all. I don't see this information online anywhere. A breakdown by PPL/CPL would be nice. I have a vague memory of someone telling me 300 but that seems a tad low for a city of 7.5 million.

Given a fleet size of 135 aircraft of Cathay Pacific, 300 pilots is definitely too low.

What makes you think Cathay pilots are licensed in Hong Kong? I know that flight training in Hong Kong is almost nonexistent due to high costs.

The training has not to take place in Hong Kong to apply for a license in Hong Kong I think. I assume that operating a commercial aircraft registered in Hong Kong requires a Hong Kong issued license, hence my previous comment.

Asking a question about a specific aspect of flying allows one of the 'experts' here to answer it. I don't intend ANY disrespect by those quote, I'm simply acknowledging by it the fact that most (or all) of the pilots/enthusiasts would (vehemenently) deny being expert at all. Asking a question requiring minutia of engineering detail would probably get Knapf to answer. :) Asking a question which does not require the above would enable any of us to do research to get your answer. Since you've already admitted to researching this, I'm not sure any of us can do any better. :D

According to an article (http://www.scmp.com/news/hong-kong/article/1524803/alarming-number-pilots-quit-hong-kong-airlines), there are "The resignations leave the airline [Hong Kong Airlines] with about 250 pilots to fly its 22 aircraft, sources said. Cathay Pacific has more than 2,900 pilots to fly about 140 aircraft."

@fooot The US has ~620,000 pilots, for a population of 319,000,000 (0.2%), if we assume a similar ratio for Hong Kong we get a little over 14,000 pilots. I suspect the truth is somewhere in between those numbers, though I have no proof. (I do know that comparatively the United States has a pretty high per capita number of pilots than most other nations, owing to our somewhat more GA-friendly training/regulatory environment & infrastructure.)

@fooot those are very interesting stats. I hadn't thought about comparing to China but it's logical! Thank you.

@DeltaLima that's exactly the reason I'm tying to find this number - I'm converting my Canadian PPL to Hong Kong and I want to know how rare I am :)

I can't answer the question, but the HK Civil Aviation Department has a form for requesting information; you might be able to get it that way.

@voretaq7 The thing that surprised me most the last time I saw the numbers for the U.S. was the number of ATPs we have. We have over 140,000 ATPs! As far as comparing HK to the rest of China... you're probably better off comparing it to, say, South Korea, Taiwan, or Japan. Hong Kong is very different economically from the majority of the Chinese population.

I reached out to the Hong Kong Civil Aviation Department and asked this question. To my surprise, I got an answer back in less than 12 hours.
As of December 31, 2014, they report:

219 private pilots
951 commercial pilots
4007 ATPL holders
20 MCPL holders

For a total of 5197 pilots licensed in Hong Kong.

Although this information isn't publicly available, but +1 for getting to a long forgotten question.

@Farhan Well it's sort-of publicly available - we have a point-in-time snapshot that Paul found for us, and information on how to contact the Hong Kong Civil Aviation Department in case anyone needs more up-to-date information


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BGBW RNP A vs Straight-in approach
Good afternoon folks. I am a bit confused and trying to find the right answer. This is what we have:

Why do we have here minimum for straight-in approach even though the descent angle is 4.1 degrees?
If we convert it to percentages it becomes around 7.2% with the groundspeed of 100 knots.
According to ICAO Doc 8168, all non-precision approaches with the gradient more than 6.5 percent for a non-precision approach for Cat A and B aircraft are considered as circle to land only. If we take a look at BGBW RNP A, it has a descent angle of 3.7 degrees, but if we measure the distance from FAF to the THR RW24 it becomes around 8.3% and now it makes sense why it’s impossible to apply straight-in minimums for RW24.
For reference, this is RNP A plate for BGBW:

The design "requirements" in PANS like Doc 8168 are more like suggestions than requirements. They have no legal force. States are free to ignore these requirements or make their own, and unlike Standards and Recommended Practices (SARPs), states are not required to specifically notify ICAO of the differences or publish them.
In other words, this appears to just be a difference from the ICAO recommendations set out in the document you mentioned. This is not unusual- there are quite a few differences between the ICAO PANS-OPS and the US TERPS as well.

@RTO Per Jeppesen, that label is specifically for aircraft handling speeds obstacle clearance in circling approaches, where PANS-OPS and TERPS differ very significantly in a way that directly impacts the safety of circling approaches and how they should be flown.


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What is the difference between the numbering of the same gate?
What is the difference and purpose of having two taxilines for the same gate where one would be numbered gate 60 and the other 60A.

(Shown above: KSLC Gate A19 with A19A besides it)

This is to allow efficient use of apron space when accommodating aircraft of different sizes. Rather than making every stand large enough for the largest possible aircraft that might use it (which would take up a lot of space), it is common to have overlapping stands where a large aircraft will essentially take up two or more stands, while smaller aircraft can park side by side.
One example: Two smaller aircraft may be parked side by side, one on stand 1L and one on 1R, blocking stand 1C. In case a larger aircraft needs to park here, it will park in the middle, on stand 1C, blocking 1L and 1R. (Or just ignore the lines altogether and park in the middle of everything, as shown on the image ...)

Similar situation here. If you look closely, you can see the central yellow line has stopping positions for larger aircraft (MD11, B747, A300, ...) while the adjacent lines have stopping positions for relatively smaller aircraft (B737, ALL PROPS, ...)

It could also be that different types of aircraft needs to park in different ways. Here, the solid yellow line is used when parking helicopters, while the dotted yellow line is for parking props.

The aircraft in your first image isn't ignoring the lines. It is parking on the white line, which is a self-positioning parking position. It allows exiting the stand under own power without needing pushback.

@Bianfable Actually the white line you see is the old markings, which have been painted over because they repositioned the stands (used to be yellow). They still used that position for a transition periode until the new markings were finished :) And you are right that the old stand allowed leaving the stand by own power


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How does the Allison (now Rolls-Royce) CX-31 centrifugal compressor achieve a pressure ratio of 10?
Centrifugal compressors typically have pressure ratios up to around 7 or 8.  But Rolls-Royce has a graphic showing the evolution of their line of small turboshaft engines that shows the compressors and their OPRs, which shows a compressor called "CX-31," which they say had a pressure ratio of TEN.
I've searched for months for any info on this compressor but have never found a single hit. Internet shows no record of this compressor being used in any engine, either production or test. Of course that could be due to any number of problems, like stall margin or production cost.
How do they achieve a pressure ratio of 10 with this compressor?

Welcome to Aviation.SE! I changed your question a bit to ask a more specific question. I hope this is what you were interested in (if not, feel free to edit again). Generic questions don't work well on this site (also see the Tour to learn more about the site).


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What do these items on a Jeppesen VOR approach chart mean?
I have some questions about certain VOR approach charts. I find some details confusing on the following chart for SLTJ VOR C Rwy 13:

What does "state" mean? It's in bold in the minima section.
Why is there a bold "1" next to the minima?
Referring to a non-precision VOR approach chart, why is the minima stated as 1DA/MDA 6950'? Shouldn't it be just MDA? The number "1" is also next to it.
Above the minima, what do the initials CDFA stand for?

The "State" label refers to the source of the values in the minimums box, in this case meaning they are from the respective state authority:

A "Std" label in the upper left corner of the minimums box indicates that the published visibilities are ICAO
Doc 9365 compliant. Other labels, as described below, indicate compliance with other regulations.
[...]
Std/State - Minimums are based on tables and rules from a State
Regulation which is similar/close to ICAO Doc 9365 (e.g. EASA AIR OPS,
Indian CAR), refer also to AIR TRAFFIC CONTROL - Aerodrome
Operating Minimums JEPPESEN for identified differences to ICAO
Doc 9365. No comparison has been done to any other landing minimums
criteria.

(Jeppesen Approach Chart Legend Minimums)

That is a footnote. You can find the note further below on the chart:

It means that using DA instead of MDA depends on your airline operating procedures.

Usually a non-precision approach uses an MDA (minimum descent altitude). But when flying a CDFA approach (see 4.), this may be replaced by a DA (decision altitude). Your airline operating procedure will stipulate how much safety margin to add to the given minima values when using them as a DA.

CDFA stands for Continuous Descent Final Approach. It means that you will not level off at MDA. Instead you start your final descent later in order to descend continuously towards the runway.


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Lighter than air vs heavier than air?
I know that hydrogen balloon is an example of lighter than air aircraft while today aircraft like helicopters and Boeing 747 are examples of heavier than air aircraft.
But I want to know about more differences in detail.
Especially, can we say that lighter than air aircraft are unpowered while heavier than air aircraft are powered?

A lighter-than-air aircraft (also called aerostat) derives lift from buoyancy, meaning its average density is lower than (or equal to) the density of the surrounding air. A heavier-than-air aircraft must derive lift by other means. This is most commonly achieved using a fixed wing combined with sufficient forward speed or a rotary wing.

Can we say that Lighter than air aircrafts are unpowered while heavier than air aircrafts are powered?

No, both types exist as powered and unpowered types, for example:

lighter-than-air
heavier-than-air

Powered
airship
aeroplane, helicopter

Unpowered
balloon
glider, gyrokite

There are also mixed types, e.g. an autogyro is a heavier-than-air aircraft with an unpowered rotary wing, but powered propeller for forward thrust.

@jkztd This is the EASA definition: "‘balloon’ means a manned lighter-than-air aircraft which is not power-driven and sustains flight through the use of either a lighter-than-air gas or an airborne heater, including gas balloons, hot-air balloons, ..."

Comments have been moved to chat; please do not continue the discussion here. Before posting a comment below this one, please review the purposes of comments. Comments that do not request clarification or suggest improvements usually belong as an answer, on [meta], or in [chat]. Comments continuing discussion may be removed.

The key distinction is that a lighter than air craft relies on aerostatics (aka buoyancy) to stay in the air, while a heavier than air craft must rely on air movement.
This means that in still air, a lighter than air craft can, in principle, stay airborne indefinitely without motive power.
In practice most lighter than air craft are un-powered balloons. It is possible to create a lighter than air craft with engines to provide propulsion and this is known as an airship. Airships have however remained relatively rare, they tend to be relatively slow, have small payloads for their size and be sensitive to wind.
A heavier than air craft on the other hand requires power to maintain altitude in still air. An unpowered heavier than air craft in still air will lose altitude. A typical glider tow apparently drops the glider off at about 1000 feet and a typical glider apparently sinks at about half a meter per second. That gives a flight time in still air of about 10 minutes.
Gliders can remain in the air for longer through exploiting rising air currents from thermals or ridge lift. However these sources of natural lift are unreliable and location dependent.

Simple, the aircraft is lighter (less dense in average) than the air surrounding it, so it floats up.
For this definition to work, the gas inside the balloon should be seen as a part of the aircraft. It may have engines, and many did. The engines are much denser than air but the whole aircraft with these engines attached is still less dense in average.

Zeppelins could produce up to 20% of their weight as dynamic lift force. They usually were a bit heavier than the air they displaced. Still, they are considered "lighter than air" (which actually is a misnomer).


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What is the purpose of this delay in the engine start procedure of the DHC-8?
For battery starting, specifically for the DHC-8 -100/-200/-300 series, there's a note in the procedure that specifies unfeathering and delaying second engine start.
The delay is specified in addition to the generator load limitations and makes reference to the FGC start-up sequence. Does anyone know what this delay accomplishes? Is there something on the secondary busses that's necessary for FGC start-up/self-test?

I'm unlikely to know the answer, but others may want to know the specific aircraft/engines to which your question refers.

@RetiredATC Aircraft was mentioned in the tags, I edited it into the question as well. Engine should be the PW123 for -100 to -300 models.

The note is in the -102/106/314 manuals, so at least PW120A/121/123E would be affected.

The Flight Guidance Computers are powered with DC off the applicable Main DC buses via feeder circuit breakers to the avionics circuit breaker panels. The FGCs also get fixed frequency 115VAC from the inverters in the nose compartment, which are running on 28vdc from the Essential and Main buses.
I can't say with 100% certainty, but it is almost certain that the FGC startup BITE depends on a stable voltage from the applicable DC bus, or possibly the TRUs can be affected by input voltage transients that can affect the 400 Hz 115VAC voltage output to the FGC.
So, running the starter/generator on the other engine in starter mode will cause voltage transients from the current load of the starter/generator, and that can mess up the FGC BITE, so they want you to wait for its BITE to complete before the running engine's starter/generator is loaded up by the opposite starter/generator.
Unfeathering brings the gas generator speed, and hence the accessory gearbox speed, up to ground idle, where it can adequately support the opposite starter/generator with current.

Interesting. Our procedures for battery start include waiting for the generator load to be below .4 (i.e. 40%; the generator load can be greater than 1.0 for up to 60 seconds after a battery start). It normally takes a couple minutes to get the battery charged enough for the DC gen load to get below .4, so I guess i wondered if that would be enough time for the FGC self-test/startup sequence. I appreciate your time considering the possibilities.

I worked on the -8 program in the early 80s and happen to have an ops manual handy to refresh my memory of the system, but this is about as far as I can go. When you have a black box and they want you to leave things alone while it does its BITE, it's almost always because of some voltage transient that affects the BITE somehow. They were probably getting faults during 2nd engine starts during testing and found if they waited it didn't happen. I suspect it has more to do with the TRU FF AC output than the DC mains.


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What problems or demands does the introduction of AIXM 5.1 solve?
I just wonder what was the benefit of introducing AIXM (Aeronautical Information Exchange Model) version 5.1+?
What problems or demands does the new version (with its complex temporary model) actually solve?
It allows some more complex scenarios, but I cannot imagine any real life use cases that were not possible in AIXM 4.5.
Might be that there are some more detailed Feature/Entity attributes. But that could be done with AIXM 4.5+?
Could anybody explain the need to me?

This version 5+ includes temporality at the feature level (in 4.5 it was at the message level). The model allows feature properties to change with time.

Source.
Version 5+ supports dynamic data. A value is associated with a validity period, and it can exist multiple values associated with different periods of validity for the same property.
From aixm.aero:

The purpose of the AIXM Temporality Concept is to enable the modelling and encoding of aeronautical data in relation with the time. As compared to the previous version 0.5, the version 1.0 (released with AIXM 5.1) includes some clarifications with regard to Schedules/Timesheets and some editorial corrections.

Typical temporary changes: Closed runway, inoperative aid, frequency change, obstacles... This example of a closure for one hour of an airport demonstrates dynamic data, and the part which was not available in 4.5:

Source.
As soon as the schedule of changes haves been entered in the database, it's easy to automatically prepare and schedule the corresponding NOTAMs. This is what is targeted by this upgrade of the model:

In order to satisfy the temporal requirements of aeronautical information systems, AIXM must include an exhaustive temporality model, which enables a precise representation of the states and events of aeronautical features. In particular, this shall enable the development and the implementation of digital NOTAM.

Yes, that's very generic and that is reflected in AIXM4.5 as well. More simple - less opportunities of different (miss)interpretations. I do not see the need for the extended model. What use case could not have been addressed before.

@m.weiloa. The point is temporality is now at the feature level, and dynamic data are available. I added an example in the answer.


text:
Are thermal anti ice systems more or less effective than deicing boots?
Are thermal anti ice systems as typically found on jet airliners more or less effective than the deicing boots typically found on turboprop aircraft such as the ATR-72?

Welcome to Aviation.SE! I removed the mentioning of the recent accident since that might inspire accident speculation, which is off-topic according to the help center. Your title question is of course perfectly on-topic.

Also make sure you understand frictional heating contributes to deice at cruse for jet airliners while not for turboprops.. 500kts vs 300kts makes a difference. Cruise altitude is another consideration. All, along with cost go into whether a design uses thermal, weeping or boots (or a combination) for deice.

Thermal anti-ice is way better.  Ice is prevented from accumulating, as opposed to accumulating and being broken off in a cycle.
Some systems are "evaporative", where the leading edge is heated to above the boiling point. You do this if the airfoil can't tolerate "runback ice" that collects behind the heated leading edge. Water droplets hitting the leading edge boil off like spitting on a steam iron.
If runback ice isn't a problem, the temperature will be lower, say 50 to 60 deg C, which reduces the thermal stress on the structure and allows less exotic materials to be used for the heating system (eg. stainless distribution tubes instead of titanium). On the CRJs, the hard leading edge CRJ200 has an evaporative system (that requires titanium distribution tubes), while the larger slatted-wing models have a non evaporative system (that can use stainless tubes).
You use boots when you are stuck with engines that can't make enough hot bleed air to heat an entire wing. Most turboprops are like that. Boots allow some ice to accumulate, and most of it is broken off when the boots inflate. It doesn't produce the clean leading edge a thermal system does, and you typically have to make adjustments to approach speeds to allow for some ice when operating with the system on.
Jets usually have the option of anti-icing the tail or not. To avoid having to anti-ice the horizontal tail, the surface is simply made larger so it can generate the required forces even with an ice load, and avoid all the plumbing to the tail.
Jets have the luxury of only dealing with ice on departures, arrivals and the related climbs and descents. Turboprops are often forced to cruise in icing.
The jet is somewhat more tolerant of severe icing than a turboprop, but if ice is severe enough you still have to get out of it ASAP. On the CRJs, icing that was bad enough to accumulate on the side windows was considered severe icing you needed to get out of.


text:
On airplanes with bleed air anti-ice systems, why is only the leading edge of the wing heated? What happens in freezing rain?
For example, if the plane is ascending or descending and there's freezing rain, surely by gravity the freezing/supercooled rain will also land on parts of the wing aft of the leading edge, will it not?

In the boundary layer, the airflow moves parallel to the wing surface:

The vertical velocity of the aircraft doesn't change the direction of the flow, it remains parallel to the wing surface.

The vertical velocity of the falling rain, 10 to 30 km/h, is negligible in this context, hence moisture moves in the same direction than air.

However there is a line where air doesn't move: The stagnation pressure line, which is on the leading edge (S):

Ice crystals accumulate here, and supercooled water hitting this line freezes. In Kutta condition, there is a symmetrical stagnation line at the trailing edge, but water cannot hit the trailing edge, the flow is moving away from it.
The pressure gradient influences ice formation too: The production of ice from water is accelerated by a pressure decrease. Along the airfoil, pressure decreases from the leading edge, and increases again after a some distance until reaching the trailing edge.
Icing is significant only at the leading edge. This is true for the empennage too, but empennage leading edges are thinner than wings, preventing ice to really build up and become a problem (apart from horns which may be electrically heated to prevent locking).

Droplets that don't impinge on the surface near the stagnation point on the leading edge tend to be carried around parallel to the surface without making contact.  However, what does actually strike the leading edge surface can flow along the surface and re-freeze where the heated area terminates. This is called "runback ice".
Some wings can tolerate runback ice and some can't. If the wing can't, you use an "evaporative" anti-ice system where the leading edge is heated above the boiling point of water, so droplets that strike evaporate away instead of running along the surface. With an evaporative system you generally don't get any icing behind the leading edge unless it's at a severe icing level, in which case you have to get out of it ASAP.


text:
Why don't jet fighters have fans?
Why don't jet fighters have fans in front? They only have a hole to suck air? I assume the compressor sucks the air. Is that enough?
See from picture below, this is an F-16 jet fighter. There is no fan. Only a hole under the cockpit to suck air. I just want to learn the reasons. Does the compressor suck air strongly?

The F-16 is powered by a General Electric F110 turbofan engine, which as the name suggests has a fan:

You cannot see the fan from the outside because the engine is located far behind the inlet below the fuselage. This image (from this answer) shows how air is directed from the inlet towards the fan:

Most (maybe all) modern fighter jets use turbofan engines (i.e. with a fan). However, turbojet engines without a fan were used on older fighters, e.g. the GE J79 used on the F-104. The compressor stages on these engines are sufficient to create airflow into the engine. The fan of a turbofan is not needed to direct air towards the engine. Its main purpose is to create accelerated bypass air, which does not go through the engine core.
There are also engines without any fans or compressors, like ramjets. However, these do not work from standstill and are therefore not practical for a fighter jet.

The F-16 has a lot of public attention, so you'll be able to (sometimes) find information via google searches. For instance, here's a video of an engine removal.

So is there an engineering reason for the extremely long intake duct on most fighters? Is it merely because they want to have the engine(s) close to the centerline but keep the intake as straight as possible? Or is there some reason that supersonic flight demands a long intake?


text:
Why is the sidestick on an Airbus so stiff?
Recently, I was so very kindly allowed to test the control surfaces on an A321. When I moved the sidestick on the Captain's side, I realised that it's FAR stiffer than my regular HOTAS stick at home. Does anybody know why it's so stiff?

Prevents bumping, maybe?

Welcome to Aviation SE! I agree with Darth's guess, but I'm not expert so I won't answer.  I will say: power steering in your car is regularly designed to self-damp as the speed of the vehicle increases, this is because even small adjustments to a vehicle in motion can result in large forces being applied to the object and its contents. I suspect this is a safety feature, meant to ensure that the captain is able to make minute changes in control input and has to work hard to make larger inputs.

lovely, thank you both ever so much

Because the force feedback from your hand is much more accurate than the position feedback. So with higher feedback forces you would know what sort of input the hand is giving without looking at it, not so with the low force levels of a games sidestick.
Way more info about the force feedback in this answer, about a stick that in first instance did not move at all: the F-16 stick. Incredibly high g-forces trying to drag the underarm of the pilot sideways, creating an ever larger input...Force feedback is what is required, as much as possible, which you get with a high spring stiffest or even a fixed stick.


text:
Why do large civil airliners use cold rather than hot thrust reversers?
Is it because cold thrust reversers are best for turbofans and most civil airliners have turbofan engines?

Smaller airplanes with both rear-mounted and under the wing engines used to have "bucket" type reversers, which captured both hot and cold exhaust. However, a bucket is not a tenable solution for large diameter engines. Fortunately, in modern turbofans the cold thrust is the vast majority of the trust, so there's no point in trying to create a bucket large enough. Therefore, axial designs became prevalent.

First off, all large civil airliners (and all civil jetliners) nowadays use turbofans, not just "most".
Second, almost all large civil airliners still in service use specifically high-bypass turbofans, with a bypass ratio1 of 5:1 or greater.  With these engines, the vast majority of the engine's thrust comes from the cold bypass air, not the hot core air, so, even if you reverse both the hot and cold streams, you'll only get a little more reverse-thrust decelerative force than you would if you reverse just the cold stream (leaving the hot core exhaust untouched).
Jetliners using low-bypass turbofans (bypass ratio under 5:1, and usually below 2-3:1) generate a larger portion of their thrust from the core exhaust (in most low-bypass engines, the core exhaust produces the majority of the engine's total thrust), so, in order for reverse thrust to generate a useful decelerative force with these aircraft, you have to reverse both the hot and the cold exhaust streams.2  However, few of these aircraft remain in service, due to the high fuel consumption and noise production of low-bypass turbofans.3
Most first-generation high-bypass-turbofan-using jetliners did have reversers for both the hot and cold exhaust streams (known as turbine reversers and fan reversers, respectively), using a number of different systems for doing so.  However, reversing just the cold bypass stream proved to have some important advantages:

On high-bypass turbofans, most systems for reversing both the turbine and fan exhausts use separate reversers for the two.  This doubles the number of mechanisms which maintenance has to keep in working order, and doubles the opportunities for things to malfunction reverser-wise.  (On the plus side, this also made it easy to remove or disable the turbine reversers while keeping the fan reversers fully operational.)
Turbine reversers have to handle the exhaust gasses from the engine core, which are:

really really hot,5 and
loaded with delightfully-corrosive combustion products and surface-eroding soot particles,
both of which cause turbine reversers to wear out much more quickly than the fan reversers,
decreasing reliability and requiring more frequent maintenance compared to the fan reversers.

Since the turbine reversers provided only a little additional decelerative force (remember, the vast majority of a high-bypass engine's thrust comes from the bypass air), deactivating them resulted in only marginal increases in stopping distances, while greatly alleviating the maintenance headaches associated with the reversers.8  Later generations of jetliners with high-bypass turbofans omitted turbine reversers entirely.

1: The ratio of the amount of air that goes through the bypass duct to the amount that goes through the engine's core.
2: Most low-bypass-turbofan-equipped airliners use(d) the iconic buckets behind the engine (known in the industry as target-type reversers), although other methods exist.
3: The only low-bypass jetliners still in relatively-common use (and mostly in third-world countries at that) are the DC-9-80 and 737-200, and even these are being retired by the day.  Low-bypass turbofans remain somewhat more common on business jets, but are slowly being superseded by the newer crop of high-bypass bizjets.  The only area in which low-bypass turbofans aren't losing ground is in military combat aircraft, where their loudness and thirst are less important than their rapid throttle response4 and their good performance in supersonic flight.
4: High-bypass turbofans have heavy, large-diameter fan rotors, which take their sweet time to get up to speed.  The smaller fans of a low-bypass engine have much less angular inertia, allowing the engine to accelerate and decelerate more quickly.
5: Even the "cold" bypass air of a high-bypass turbofan is still quite hot by human standards, due to the considerable amount of essentially-adiabatic6 compression produced by the whirling fan (which is also known as a low-pressure7 compressor; not only does it fling air down the bypass duct, it also helps precompress the air going into the core); however, the core exhaust, having been subjected to compression not only from the fan but also from all the other compressor stages, and then used to burn fuel in the combustion chamber(s), is much hotter still.
6: Adiabatic compression is compression that occurs quickly enough that there's no time for heat to leak into or out of the stuff being compressed, which is a pretty-good approximation for what happens to the bypass air in a turbofan.
7: "Low-pressure", here, is very relative.
8: In this respect, the story of turbine reversers is reminiscent of that of nosewheel braking - another deceleration system fitted to some early jetliners, found to be more trouble to maintain than the marginal improvement in stopping distances it provided could justify, and, consequently, disabled or removed.

https://xkcd.com/1208/

My friend is typing in ATR-72 right now, which most certainly is an airliner, but does not use turbofan engines. Maybe change that "all civil airliners" to "all large civil airliners"?

@zaitcev: Whoops, was thinking of jetliners (where the use of turbojets is extinct).  Fixed.


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How does afterburning not affect upstream air in the engine?
I've read in many books that afterburners are designed not to affect the upstream engine components, so if there's a large amount of combustion going on after the addition of afterburner fuel, how does the reaction of the air to the sudden acceleration not force air back into the turbine stages?

How would this affect upstream components? There’s more combustion occurring, so there’s more air needed. This should pull air AFT instead of forcing it forward.

@MD88Fan Please don't answer in the comments, as this bypasses quality control (the infamous downvote). More combustion does not equal "pulling" air - a jet engine is not comparable to natural convection.

The afterburner will always affect the upstream flow, as it is subsonic throughout the entire engine (remember, hot air has a higher speed of sound). A properly designed afterburner should therefore not significantly increase the pressure upstream, because this may result in a compressor stall.
Typically this is solved by increasing the nozzle opening area, so that the nozzle back pressure is reduced. Variable geometry nozzles are the norm for reheat capabilities. This in turn keeps the pressure at the turbine outlet roughly the same. Still, most engines require to be at some minimal power level before igniting the afterburner.

It's pretty simple. When in reheat, the core engine is already running at maximum thrust, so the inertia of the mass flow out the turbine section is such that the reheated air has nowhere else to go but out the back.
The afterburner is more or less a ramjet, except with the main engine's exhaust doing the ramming in place of high airspeed.


text:
What are lateral, longitudinal and directional stability?
When we talk about aircraft having a stable configuration, it refers to lateral, longitudinal and directional stability. Can someone give a practical explanation of what those are and how they work?

The answer here is found in the Pilot's Handbook of Aeronautical Knowledge (and probably elsewhere) and is as follows:

The longitudinal axis of an aircraft is more or less a straight line through the aircraft's nose cone or prop hub and the endpoint of the fuselage (the aircraft's center of gravity will usually lie along or just slightly above/below this line as well). It is the axis around which the aircraft rolls, controlled by the ailerons. The lateral axis is parallel to the wings and passes through the aircraft's center of gravity. It is the axis around which the aircraft pitches, as controlled by the elevators. Finally, the vertical axis is "normal" (perpendicular in all directions to) the geometric plane formed by the longitudinal and lateral axes, parallel to the aircraft's primary lift vector and (in level flight) its weight vector. It is the axis around which the aircraft yaws, controlled by the rudder.
Rotation about any one axis is the job of one linked set of control surfaces as mentioned above. Stability in that same axis, roughly defined as keeping that line passing through your aircraft pointing in the same direction, is the job of the other two surfaces in concert, but primarily the one that would move the endpoints of that axis up or down relative to the aircraft. So, elevators provide lateral rotation to achieve longitudinal stability, and vice versa for ailerons.
It differs slightly for the vertical axis, as if your plane is both longitudinally and laterally stable, it's also "vertically stable", however the plane is longitudinally and laterally stable, but completely out of control, in a "flat spin". Stability in the vertical axis therefore is secondary to "directional stability", which keeps the longitudinal axis pointing in a particular direction along the geometric plane formed by the lateral and longitudinal axes. In this case the rudder both controls yaw and provides directional stability.
In addition to control surfaces, weight and especially the aircraft's center of gravity is important in stability. Ideally, most small planes are most stable in level flight when the CG of the aircraft is exactly on the centerline of the aircraft (between the tip of the nose and the tip of the tail) and slightly forward of the aircraft's center of lift (which depends on the wing's angle of attack but is usually close to the thickest point in the wing's cross-section). In this configuration, while the aircraft is travelling forward normally, downwash from the wings flow over the top of the horizontal stabilizer, keeping the nose level. In a stall, the slight nose-heavy configuration along with the stabilizers in the rear will cause the nose to point downward gently, restoring normal airflow and allowing the pilot to recover.
If the CG is too far forward, the pilot will need to apply upward pitch or trim to keep the nose level. This will reduce the amount of travel he will have available to pitch up, and in a stall, the plane will nose down sharply and the elevator may provide insufficient force for the pilot to pull out of the dive.
If the CG is too far aft, the plane will want to nose up constantly, and the pilot will have to apply downward elevator or trim. In a stall, a plane with an aft CG won't nose down, preventing the pilot being able to restore normal airflow over the wings. This is especially dangerous in an uncoordinated stall aka a spin; the aft center of gravity in combination with the forward thrust of the engine will "stabilize" the plane in the spin and make it impossible to recover.
If the CG is off the aircraft's centerline, the plane will tend to roll towards its heavier side. This is compensated for with ailerons or aileron trim, and for most everyday flight it's the easiest to compensate for, but it can cause unfamiliar roll behavior and a tendency to spiral down, which the pilot must be aware of and correct for.

Lateral stability is roll stability: the tendency of the aircraft to reduce its rolling and return to an upright position unless continually maintained in position by e.g. the ailerons. (This is usually only partial.)
Longitudinal stability is pitch stability: the tendency of the aircraft to reduce its pitching and return to a level position (relative to the direction it's traveling, at least) unless countered by e.g. the elevators.
Directional stability (also known as vertical stability) is yaw stability: the tendency of the aircraft to reduce its yawing and return to a straight position (relative to the direction it's traveling, at least) unless countered by e.g. the rudder.
All these are necessary to avoid having to continually make tiny, precise adjustments to all the controls.

in summary: Longitudinal stability: Pitch (about lateral axis) → elevators, Directional stability: Yaw (about vertical axis) → rudder, Lateral stability: Roll (about longitudinal axis) → ailerons.


text:
Where can I get the FAA data that defines airspace, including MOAs etc.?
I was looking at http://www.gelib.com/aeronautical-charts-united-states.htm, where you can download shape files for Google Earth that show US airspaces.  I'm writing some software that has a similar need and need to find a source for this data.  I'm looking for data that defines the extents of airspaces including MOAs, restricted areas, etc.
I have been pouring through the FAA's website with no luck.  The link I referenced above says its source was the National Aeronautical Charting Office (NACO), which I'm having very little luck finding as well.  I think it may have been renamed, thus the poor results.  I also called the FAA and can't seem to find anyone there that knows where to transfer me.
So, does anyone here have any helpful pointers on finding said information?  I want to pull it directly from the source to make sure it is always up-to-date and accurate.

I hope you are making an open-source data card that would free people from the grip of Jepp updates.

can you actually download shape files from the gelib.com URL you gave?  I've tried, and don't see anything in GoogleEarth.

No.  I was able to download some shape files from the link 2NinerRomeo provided in his answer below but they appear to be 2 dimensional shape files.  I'm still looking into that because I don't have much experience working with shape files... maybe they are 3d and I just haven't found the "magic button" in the viewer yet.

Yeah, I don't think it works.... but I'm looking at some of the data from the other sources, and I'm going to try to change it into GoogleEarth KML files.  If I'm successful, I'll followup here.

Excellent, I'm more interested in the data you use to produce the KMLs than the KMLs themselves so any insight you can provide would be appreciated.

@mins: Sure enough.  I've deleted the comment.

search for 3D Google Earth Airspace Lloyd Bailey

Take a look at the National Flight Data Center.  The data appears to be contained in NASR. I mentioned in my comment that there may be no official source for spatial data, but I think this may include what you are looking for.
These are derived products. The cannonical definitions are the legal descriptions found in FAA Orders with the prefix 7400. At the time of this writing, JO 7400.11 (current version E) covered airspace classes A,B,C,D and E as well as reporting points. The JO 7400.10 (current version C) covered special use airspaces. Both publications can be found at the FAA's Website.

Found some good stuff here, thanks.  My initial opinion is that the airspace files won't suit my needs but there was a lot of other good information there that I needed as well.  Appreciate the link.

Are the 2D shapefiles the canonical master definitions or derived products? For Class C airspace, for example, does the FAA maintain exact definitions of a cylinder with a given center, radius, and altitude within one or more radarcs having a certain center, inner and outer radii, start and stop radials, etc.?

I would call these derived products.  The cannonical definitions would be the legal descriptions found in FAA Orders with the prefix 7400. At the time of this comment, order 7400.9Z covered Classes A,B,C,D,E as well as reporting points.  7400.8X covered special use airspaces.,  Both can be found here: http://www.faa.gov/air_traffic/publications/#orders  7400.9Z superseded 7400.9Y.  These are published yearly.  I'm not sure what will come after .9Z.

You are looking for Aeronautical Navigation Products (AeroNav Products) (formerly NACO).
They have a number of digital products, which should have the data that you are looking for.
If not, their contact information is:

Customer Service:  (800) 626-3677
  9-AMC-Aerochart@faa.gov
Abigail "Abby" Smith, Director
  FAA, AeroNav Products AJV-3
  1305 East-West Hwy
  Silver Spring, MD 20910
  (301) 427-5000

Thanks, I have left a message with them and will post a comment here if I get any good information from them.

You may take a look at openAIP. It contains nearly up-to-date airspace and airport information besides other aviation topics. Airspace data is available for download as .aip file. This is actually a very simple XML file that can easily be used in any application.

OpenAIP is a pretty cool project, but they describe it as a "crowd-sourced aeronautical information platform that allows registered users to add, edit and download aeronautical data" -- that strikes me as having a big red NOT FOR NAVIGATION - USE OFFICIAL CHARTS FOR FLIGHT stamp on it :-)

Or take a look at Open Flightmaps, map and raw data as OFMX (a slight variation of AIXM 4.5) as well as a few other formats available.

The current information can be downloaded at:
https://nfdc.faa.gov/xwiki/bin/view/NFDC/56+Day+NASR+Subscription
This includes the digital vector copies of the sectionals and shape files for the airspaces.

FAA's Coded Instrument Flight Procedures.

Data elements included are: Airports and Heliports, Runways, VHF, NDB and ILS Navigation Aids, Waypoints (Terminal and Enroute), Airways, Off Route Obstruction Clearance Altitude (OROCA) records, Departure Procedures (DPs), Standard Terminal Arrival Procedures (STARs), Special Use Airspace (SUAs) and Class B, C, and D Airspace. Also included are GPS, RNAV (GPS), RNAV (RNP), GPS Overlay, ILS, LOC, LDA and SDF Standard Instrument Approach Procedures (SIAPs) with their associated Minimum Safe Altitude (MSA) data.

SUAs most likely includes MOAs.
It's encoded in ARINC 424. The specification is not free but can be found using a search engine.


text:
Can I log simulated IMC time in a simulator without a CFII?
I am working on my instrument rating under part 61. I need 40 hours of simulated IMC and 20 of those hours can be in a certified AATD simulator like a redbird. Do I need a CFII to sign off on those hours in the redbird or can I log those hours myself and count them towards my 40 hours of sim IMC?

Welcome to aviation.SE! I added the faa-regulations tag to your question; please always tell us which country or regulations you're asking about.

There is a difference between a Basic Aviation Training Device (BATD) and an AATD. You can log time for the instrument rating in the old-style BATD but cannot use it for maintaining instrument currency. Refer to AC 61-136B for details.

Yes, you need an instructor. 14 CFR 61.65(i) (emphasis mine):

Use of an aviation training device.  A maximum of 10 hours of
instrument time received in a basic aviation training device or a
maximum of 20 hours of instrument time received in an advanced
aviation training device may be credited for the instrument time
requirements of this section if -
(1) The device is approved and authorized by the FAA;
(2) An authorized instructor provides the instrument time in the
device; and
(3) The FAA approved the instrument training and instrument tasks
performed in the device.

You may be thinking of maintaining IFR currency using a simulator. That doesn't need an instructor, per 61.57(c)(2).

You can log it, but it won't count towards a certification

A subtle, detail, @RonBeyer. Subtle, but very important!


text:
What are the V speeds for a Piper Super Cub 180HP with an upgrade to 61 gallons fuel capacity?
I fly a Piper Super Cub 180HP with upgraded 61 gallons, 30.5 gallons usable a side. The POH and everywhere I look online I find POHs for Cubs with 150hp and 36 gallons TOTAL. I was wondering if anyone knew where to find the speeds for this upgraded Super Cub?

Isn't this part of the STC documentation when the mod was performed? I believe it's required to be an addendum to the POH...

There should be an addendum to the POH, however, the other flight characteristics didn't change, so you could calculate them yourself using a current weighing report.

Before you even get that far, does the STC for the bigger engine and extra fuel include a gross wt increase? (usually this is not the case because it would require the STC maker to do a ton of testing)  If there is no GW increase, you will use the regular numbers. Or, perhaps this airplane is a Wag Aero homebuilt Super Cub?  If that's the case, the builder should have created a POH and if not you are on your own.


text:
What is the difference between "cruise" and "en-route"?
I have a little misunderstanding about the two concepts: Cruise and En-route.
Could you guys help me to compare the similarities and differences between these two concepts, maybe in some aspect like air traffic controller or aircraft performance, flight mechanics, ...
Its would be my pleasure to see some answer from you guys, thanks a lot.

Enroute and cruise are essentially the same thing. Why do you think they are different?

I think ''En-route''  is larger in meaning than ''Cruise''. Am I right ? But how about in air traffic control ? Did ATC used these two as one ? I normally heard they used cruise more often than ''en-route''

“Cruise” implies a power setting. Most aircraft use “cruise” power to be “enroute”. Long range cruise, economy cruise, high speed cruise, etc. There might be some rare exceptions I suppose.

Where have you heard these terms? What is the context?

Where have you heard these terms used? What is the context?

En-route, often written enroute has been always understood as the phase where the aircraft has completed the departure procedure and has not yet transitioned to the arrival procedure. During this phase aircraft are flying along "airways", medium and high altitude corridors where aircraft are "cruising", and are controlled by specific control centers.
But in order to make sure all countries and aviation organizations use the same wording, ICAO wanted to create a precise definition. The first goal was to have a common classification of incidents and accidents. ICAO enroute phase includes cruising:

Instrument Flight Rules (IFR):  From completion of Initial Climb through cruise altitude and completion of controlled descent to the Initial Approach Fix (IAF).

Visual Flight Rules (VFR): From completion of Initial Climb through cruise and controlled descent to the VFR pattern altitude or 1,000 feet above runway elevation, whichever comes first.

Source: ICAO.
Anything using a wording like "whichever comes first" is to be taken for serious imho :-)
Enroute
Some more details:

This phase of flight includes the following subphases

Climb to Cruise: IFR: From completion of Initial Climb to arrival at initial assigned cruise altitude. VFR: From completion of Initial Climb to initial cruise altitude.
Cruise:  Any level flight segment after arrival at initial cruise altitude until the start of descent to the destination.
Change of Cruise Level:  Any climb or descent during cruise after the initial climb to cruise, but before descent to the destination.
Descent: IFR:  Descent from cruise to either Initial Approach Fix (IAF) or VFR pattern entry. VFR: Descent from cruise to the VFR pattern entry or 1,000 feet above the runway elevation, whichever comes first.
Holding: Execution of a predetermined maneuver (usually an oval racetrack pattern) which keeps the aircraft within a specified airspace while awaiting further clearance. Descent during holding is also covered in this subphase.

Source: ICAO.
There are IFR Enroute Aeronautical Charts, used for the enroute phase of the flight, and there is an enroute control:

Source: What is a TRACON?
Cruise
Cruise is flying at a certain engine economical regime, when the goal is mostly to cover the distance to the destination. Other phases include complying with noise regulations, using safe paths in busy areas, climbing, descending. During these phases the aircraft is under-performing from fuel consumption and speed standpoints.
Engine and aircraft manufacturers give to cruise a certain meaning, e.g. engines are optimized for cruise, this is where they are most efficient.

Airbus, cruise parameters page, Source
Aircraft are cruising in a clean configuration, that is all drag sources have been retracted. From Airbus:

Source

As others have said, the terms are mostly synonymous when referring to an aircraft's phase of flight, with the meaning "the aircraft has leveled off at their highest desired altitude." But as there are a few specific meanings:

Like @757toga mentioned, a cruise clearance is a clearance for an aircraft to fly at any appropriate altitude along their route toward their destination airport, and then execute any approach procedure at that airport. It is important to be aware of that terminology and not mistake another term for a "cruise clearance."

So it would be unlikely for a controller do describe an aircraft as "cruising" unless they had been issued a cruise clearance. Instead they would describe the traffic as "enroute at [altitude]" or "level at [altitude]."

FAA facilities are divided into Terminal (towers and low-altitude Terminal Radar Approach Controls) and Enroute (high-altitude Air Route Traffic Control Centers). The term "cruise facility" would never be substituted for "enroute facility."

In other countries low-altitude facilities are called terminal control and high-altitude facilities are called area control.

As @Mike mentioned, aircraft documents are more likely to refer to "cruise power setting" rather than "enroute power setting."

In general, you will need to be more specific about your context in order to see what exactly the terms mean and if there is any difference between them.

important to mention: Your first bullet point is only true for FAA-Land. The rest of the world doesn’t know a „cruise-clearance“ and would usually talk about aircraft „cruising at FLXXX“ or „requested cruise level“.


text:
Why do twin-engine Diamond aircraft have a single FADEC lever instead of two engine throttles?
From my understanding, if you have a twin-engine aircraft with 2 lever engine throttle one for each engine you will be able to use differential thrust to steer in case of steering malfunction.
I don't understand why twin-engine Diamond aircraft decide to go with only single lever FADEC.
Here is the video I encounter that lead to this question.

The DA 42 and DA 62 aircraft do not have throttle levers, they have power levers.  And the DA-42 and 62 have two power levers, one for each engine.  See the picture of the DA-62 cockpit below.

The cockpit images that you see there in the video are from the DA 40 NG airplane, which uses a single power lever for its turbo Diesel engine.
The reason for including two power levers is 1) independent control of engine power for each engine in the event of an engine failure 2) use of differential thrust for slipping in crosswind landings and 3) differential thrust to assist while turning on the ground.

@JanHudec commented under another (now-deleted) answer -- "Note that even non-FADEC engines would only have one power lever rather than throttle + mixture that gasoline engines have, because diesel does not have throttle and only meters fuel. On gasoline engine FADEC could automatically control mixture to dispense with it too, but on diesel it mainly prevents stalling the engine by abrupt power lever movement and adjusts timing to improve efficiency."

This answer leads us back to this tangentially-related question -- https://aviation.stackexchange.com/questions/50486/how-can-using-split-throttles-help-when-landing-twins-in-crosswinds


text:
What is the effect of aspect ratio on cruise velocity?
Does the Aspect ratio affect the Maximum cruise velocity please?
I was thinking it wouldn’t as Aspect ratio= b^2/S.
If I wanted to go quicker I could just increase my span and decrease S?

The key is total drag.  At some point extra form drag and extra weight from structural strength requirements cancel reduction in induced drag.  Interesting that undersea creatures swimming in greater density water do not sport very high AR.

You should explain what variables you are using.  So, is "b" span, and "S" area?

So basically it seems you are suggesting to cruise as fast as possible, we should make the aspect ratio as high as possible, and to scale down the wing (reduce area) as needed to allow us to operate at an optimal angle-of-attack during high-speed cruising flight. (Obviously a high aspect-ratio wing is a poor choice for operating at a low angle-of-attack.)  Well, that all seems plausible --


text:
Is there somewhere I can download real FDR data?
Is there any way to get FDR data via the internet? Like actual FDR data from 9/11:
https://undicisettembre.blogspot.com/2007/07/ntsb-flight-recorder-data-and.html

The only cases I'm aware of where you can do that is when the NTSB publishes it in an accident report. Otherwise, that data is considered proprietary to the aircraft operator & isn't disclosed to the public.

In which case I've seen it reported in graph format, but not as raw data.


text:
Is there a reason for a jet liner to raise the nose wheel early during the take-off run?
This video shows a 727 apparently rotating too soon.
The take-off run starts at 2:00 minutes and at 2:11, you can see the yoke is pulled back by the elevators rising.
It seems to me that this is no mistake and that the early pull, held until the main wheels leave the ground and accelerating with the nose wheel held off, is very deliberate.
Is there a solid operational reason for doing this?

@KorvinStarmast.  I might have missed something but I can clearly see the 3rd engine and there are 727 variants with winglets.

The first video link and the second are not the same.  I clicked on "this video" and saw a 737 from the rear.  I clicked on 2:00 and found a 727.  My mistake.

@KorvinStarmast All three links are to the same video and the edit history shows that the links have never been changed.

@DavidRicherby All I can tell you is that the first link got me an 11 minute video that, at two minutes in, was a different video than the other one, hence my original comment.  It the well known "cross winds from hell on an  uneven runway" video.

The only reason I can think of offhand would be that the airplane had a nose gear shimmy, and the flying pilot elected to stop it by getting it off the runway early. Of course, any such shimmy should be written up for maintenance attention.

@Terry, I think it's much simpler: just a jigit showing off. It's Azerbaijan Airlines!

This is a take off maneuver which allows the aircraft to take off at the minimum speed possible, called V2min. It presents the best possible airfoil attitude to the air to generate lift at a low speed.
You can see the same thing being done with the Airbus A380 during its flight testing.


text:
Is the 250kt limit below 10,000 feet MSL varied according to airfield height?
FAR part 91 Sec 91.117 for example states that "unless otherwise authorized by the Administrator, no person may operate an aircraft below 10,000 feet MSL at an indicated airspeed of more than 250 knots".  All of the major jurisdictions I can find reference to have the same rule.
I've highlighted MSL (mean sea level).  Why is MSL used?  What's the point of such a regulation when aircraft would be able to manoeuvre at high speed close to the surface of high elevation fields?
How, if at all, is this varied when operating to or from an airfield with a high elevation, for example Quito, Ecuador (SEQM) at 7,910 feet?

If it was to vary then the regulation would have said AGL wouldn't it?

@DeepSpace I wasn't asking about that specific rule and this forum is usually polite and free of sarcasm which is neither wanted nor appreciated.  What is the point of having a 10,000 foot rule when it would only apply for a couple of thousand feet above high elevation fields such as SEQM.  Is the intent of my question not clear?  Do you have a proposed edit?

Related, but not the answer: Origin of the 250 knots below 10,000 ft rule. Also, for EU: Did this aircraft illegally exceed 250kts below 10,000ft?

@mins Hi. I think I haven't been clear.  The rule is established to aid sequencing and separation.  These regs mean that an aircraft could be manouvering in class A at high speed only 3000 feet about the surface.  Surely there must be something to deal with this?

If you aren't asking about how the rule is interpreted and don't want people to misunderstand, you may want to reword your title which asks "IS the ... varied according to...", which is clearly "no, it isn't"...

I'm a bit confused by your question and comments. First, you quoted 117(a) but 117(b) has speed limits that apply above class C/D airspace based on AGL, not MSL; that may answer part of your question. Second, which jurisdiction are you asking about? Part 91 is irrelevant in Ecuador, and class A airspace in the US starts at 18,000MSL so to be "3000 above the surface" would require an airport elevation of 15,000MSL, but the highest one in the US is at 9,927.

First, note that the regulation applies to BELOW 10K ft, not to what may or may not be done above that altitude.  Second, it's far easier to measure altitude from MSL rather than AGL.  That's pretty hard to do without something like a radar altimiter, and anyway is subjuct to abrupt fluctuations as an aircraft passes over terrain.

No, the 250 KIAS below 10000 MSL rule of 91.117(a) is not varied within FAA jurisdiction, including near airports with high field elevation.
The 200 KIAS near Class C and D airfields rule of 91.117(b) mitigates some of the risk you might perceive surrounding high elevation airports.
For example, Aspen (KASE) has a surveyed field elevation of 7838 ft and is within Class D airspace. Aircraft operating within 4 NM of Aspen would be speed limited to 200 KIAS up to 2500 AGL, which would equate to over 10000 MSL.
Other high elevation airfields in Class E or G airspace would presumably have less traffic, with the expected result that higher indicated airspeeds would pose less risk in the uncongested airspace..